Bipolar junction transistors as switches - ibiblio · Bipolar junction transistors as switches ... Some of the following transistor switch circuits are properly conﬁgured, and some

Bipolar junction transistors as switches

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1

Question 1

Solid-state switching circuits usually keep their constituent transistors in one of two modes: cutoff orsaturation. Explain what each of these terms means.

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Answer 1

”Cutoff” refers to that condition where a transistor is not conducting any collector current (it is fullyoff). ”Saturation” means that condition where a transistor is conducting maximum collector current (fullyon).

Follow-up question: is there such a thing as a state where a transistor operates somewhere betweencutoff (fully off) and saturation (fully on)? Would this state be useful in a switching circuit?

Notes 1

In all fairness, not all transistor switching circuits operate between these two extreme states. Some typesof switching circuits fall shy of true saturation in the ”on” state, which allows transistors to switch back tothe cutoff mode faster than if they had to switch back from a state of full saturation. ECL (Emitter-CoupledLogic) digital circuits are an example of non-saturating switch circuit technology.

2

Question 2

Explain the function of this light-switching circuit, tracing the directions of all currents when the switchcloses:

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Answer 2

All currents shown usingconventional flow notation

Notes 2

Ask your students to explain what possible purpose such a circuit could perform.

3

Question 3

Trace the directions of all currents in this circuit, and determine which current is larger: the currentthrough resistor R1 or the current through resistor R2, assuming equal resistor values.

R1

R2

SW1

SW2

If switch SW2 were opened (and switch SW1 remained closed), what would happen to the currentsthrough R1 and R2?

If switch SW1 were opened (and switch SW2 remained closed), what would happen to the currentsthrough R1 and R2?

file 00522

Answer 3

I’ll let you determine the directions of all currents in this circuit! Although it is impossible to tell withabsolute certainty, the current through R1 is likely to be much greater than the current through R2.

If SW2 opens while SW1 remains closed, both currents will cease. If SW1 opens while SW2 remainsclosed, there will be no current through R1, but the current through R2 will actually increase.

Follow-up question: what does this indicate about the nature of the two currents? Which current exertscontrol over the other through the transistor?

Notes 3

The most important principle in this question is that of dependency: one of the transistor’s currentsneeds the other in order to exist, but not visa-versa. I like to emphasize this relationship with the wordscontrolling and controlled.

4

Question 4

Calculate all component voltage drops in this circuit, assuming a supply voltage of 15 volts, an emitter-base forward voltage drop of 0.7 volts, and a (saturated) emitter-collector voltage drop of 0.3 volts:

file 02050

Answer 4

15 V

0.3 V

14.7 V

0.7 V

0.4 V

14.3 V

Notes 4

An interesting point that some students may bring up is the 0.4 volt drop across the base-collectorjunction. Attentive students will note that this junction is supposed to be reverse-biased, but Kirchhoff’sVoltage Law clearly specifies the polarity of that 0.4 volt drop, and it is in the direction one would expect forforward biasing of that junction. An examination of the energy diagram for a conducting bipolar junctiontransistor is really necessary to explain why that junction is considered to be reverse-biased.

5

Question 5

A student attempts to build a circuit that will turn a DC motor on and off with a very delicate (lowcurrent rating) pushbutton switch. Unfortunately, there is something wrong with the circuit, because themotor does not turn on no matter what is done with the switch:

Mtr

This circuit does not work!

Correct the error(s) in this circuit, showing how it must be set up so that the transistor functions asintended.

file 02047

Answer 5

Mtr

Notes 5

It is very important for your students to learn how the base current controls the collector current ina BJT, and how to use this knowledge to properly set up switching circuits. This is not difficult to learn,but it takes time and practice for many students to master. Be sure to spend adequate time discussing thisconcept (and circuit design techniques) so they all understand.

6

Question 6

A stepper motor is a special type of electric motor typically used for digital positioning systems. Themotor shaft rotates by alternately energizing its coils in a specific sequence. These electromagnet coils drawa fair amount of DC current (several amperes is not uncommon for heavy-duty stepper motors), and as suchare usually triggered by power transistors:

Stepper motor

5 V@ 2 A

(from control circuit)(To other transistors)

The control circuit that usually sends pulse signals to the base of the power transistors is not shown inthis diagram, for simplicity. Your task is to draw a pushbutton switch in this schematic diagram showinghow the first motor coil could be manually energized and de-energized. Be sure to note the directions ofcurrents through the transistor, so that your switch is installed correctly!

Also, explain the purpose of the diode connected in parallel with the motor coil. Actually, there will beone of these diodes for each of the motor coils, but the other three are not shown for the sake of simplicity.

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7

Answer 6

Stepper motor

5 V@ 2 A

(To other transistors)

The diode prevents damage to the transistor resulting from inductive ”kickback” each time the motorcoil is de-energized.

Notes 6

It is very important for your students to learn how the base current controls the collector current ina BJT, and how to use this knowledge to properly set up switching circuits. This is not difficult to learn,but it takes time and practice for many students to master. Be sure to spend adequate time discussing thisconcept (and circuit design techniques) so they all understand.

8

Question 7

Some of the following transistor switch circuits are properly configured, and some are not. Identifywhich of these circuits will function properly (i.e. turn on the load when the switch closes) and which ofthese circuits are mis-wired:

Circuit 1 Circuit 2

Circuit 3 Circuit 4

Load Load

Load

Load

file 02326

9

Answer 7

Circuit 1 Circuit 2

Circuit 3 Circuit 4

Load Load

Load

Load

This will work! This will work!

This circuit is bad This circuit is bad

Notes 7

This is a very important concept for students to learn if they are to do any switch circuit design – atask not limited to engineers. Technicians often must piece together simple transistor switching circuits toaccomplish specific tasks on the job, so it is important for them to be able to design switching circuits thatwill work.

Have your students describe to the class how they were able to determine the status of each circuit, sothat everyone may learn new ways of looking at this type of problem. Also have them describe what wouldhave to be changed in the ”bad” circuits to make them functional.

10

Question 8


Circuit 1 Circuit 2

Circuit 3 Circuit 4

Load

Load

Load

Load

file 02327

11

Answer 8

Circuit 1 Circuit 2

Circuit 3 Circuit 4

Load

Load

Load

This will work!

This will work!

This circuit is bad

This circuit is bad

Load

Notes 8



12

Question 9


Circuit 1 Circuit 2 Circuit 3


file 02409

13

Answer 9



This circuit will work! This circuit is bad This circuit is bad

This circuit will work! This circuit is badThis circuit is bad

Follow-up question: circuit #3 is different from the other ”bad” circuits. While the other bad circuits’lamps do not energize at all, the lamp in circuit #3 energizes weakly when the pushbutton switch is open(not actuated). Explain why.

Notes 9



14

Question 10

In each of the following circuits, the light bulb will energize when the pushbutton switch is actuated.Assume that the supply voltage in each case is somewhere between 5 and 30 volts DC (with lamps andresistors appropriately sized):



However, not all of these circuits are properly designed. Some of them will function perfectly, but otherswill function only once or twice before their transistors fail. Identify the faulty circuits, and explain whythey are flawed.

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Answer 10

Circuits 3, 5, and 6 are flawed, because the emitter-base junctions of their transistors are overpoweredevery time the switch closes.

Hint: draw the respective paths of switch and lamp current for each circuit!

Notes 10

This is a very important concept for students to learn if they are to do any switch circuit design – atask not limited to engineers. Technicians often must piece together simple transistor switching circuits toaccomplish specific tasks on the job, so it is important for them to be able to design switching circuits thatwill be reliable. A common mistake is to design a circuit so that the transistor receives full supply voltageacross the emitter-base junction when ”on,” as three of the circuits in this question do. The result is suredestruction of the transistor if the supply voltage is substantial.

Circuit #3 is a tricky one! The presence of a resistor might fool some students into thinking base currentis limited (as is the case with circuit #2).

15

Question 11

Draw the necessary wire connections so that bridging the two contact points with your finger (creatinga high-resistance connection between those points) will turn the light bulb on:

Contactpoints

file 00445

Answer 11

Contactpoints

Notes 11

Once students learn to identify the two current paths (base versus collector), especially the properdirections of current for each, the interconnections become much easier to determine.

Some students may place the light bulb on the emitter terminal of the transistor, in a common-collectorconfiguration. This is not recommended, since it places the light bulb in series with the controlling (base)current path, and this will have the effect of impeding base current, and therefore the controlled (light bulb)current. Given the very high electrical resistance of human skin, this circuit needs all the gain we can possiblymuster!

This circuit works well if an LED is substituted for the incandescent lamp.

16

Question 12

The ignition system of a gasoline-powered internal combustion automobile engine is an example of atransformer operated on DC by means of an oscillating switch contact, commonly referred to as the contact”points”:

Chassisground

"Coil"

Cam

Sparkplug

Ignition switch

12 VDC

"points"

The cam-actuated ”point” contacts open every time a spark is needed to ignite the air-fuel mixture inone of the engine’s cylinders. Naturally, these contacts suffer a substantial amount of wear over time due tothe amount of current they must make and break, and the frequency of their cycling.

This device was seen by automotive engineers as a prime candidate for replacement with solid-statetechnology (i.e., a transistor). If a transistor could take the place of mechanical ”point” contacts for makingand breaking the ignition coil’s current, it should result in increased service life.

Insert a transistor into the following circuit in such a way that it controls the ignition coil’s current,with the ”point” contacts merely controlling the transistor’s state (turning it on and off):

17

Chassisground

"Coil"

Cam

Sparkplug

Ignition switch

12 VDC

"points"

file 00447

18

Answer 12

Chassisground

"Coil"

Cam

Sparkplug

Ignition switch

12 VDC

"points"

Follow-up question: assuming the primary winding of the ”coil” has an inductance of 9 mH an a wireresistance of 0.4 Ω, determine the amount of time necessary to build to full current once the transistor orpoints have turned on (after 5 time constants’ worth of time).

Notes 12

Ask your students, ”what is the purpose of the capacitor in this circuit?” Inform them that without it,the points would have ”burned up” very quickly, and that the transistor will fail almost immediately!

Some of your students familiar with engine ignition systems will notice that there is no distributor formultiple spark plugs. In other words, this circuit is for a single-cylinder engine! I chose not to draw adistributor in this schematic just to keep things simple.

19

Question 13

Electronic ignition systems for gasoline-powered engines typically use a device called a reluctor to triggerthe transistor to turn on and off. Shown here is a simple reluctor-based electronic ignition system:

Chassisground

"Coil"

Sparkplug

Ignition switch

12 VDC

N S

Reluctor

Explain how this circuit functions. Why do you think the triggering device is called a ”reluctor”? Whatadvantage(s) does this circuit have over a mechanical ”point” operated ignition system?

file 00448

Answer 13

The ”reluctor” generates pulses of current to the transistor’s base to turn it on and off. The word”reluctor” is applied to this device in honor of a certain magnetic principle you should know!

Notes 13

Discuss the advantages of a reluctor-triggered ignition system with your students. As far as I am aware,the system possesses no disadvantages when compared against mechanical point-driven systems.

An interesting side note: one method of testing a reluctor-driven ignition system at high frequencieswas to hold the tip of a soldering gun (not a soldering iron!) next to the pickup coil and pull the trigger.The strong magnetic field produced by the gun’s high current (60 Hz AC) would trigger the ignition systemto deliver 60 sparks per second.

Some of your students familiar with engine ignition systems will notice that there is no distributor formultiple spark plugs. In other words, this circuit is for a single-cylinder engine! I chose not to draw adistributor in this schematic just to keep things simple.

20

Question 14

Predict how all component voltages and currents in this circuit will be affected as a result of the followingfaults. Consider each fault independently (i.e. one at a time, no multiple faults):

Chassisground

"Coil"

Sparkplug

Ignition switch

12 VDC

N S

Reluctor

Q1

C1

T1

L1

V1

• Transistor Q1 fails open (collector-to-emitter):

• Transistor Q1 fails shorted (collector-to-emitter):

• Reluctor magnet weakens:

• Capacitor C1 fails shorted:

• Capacitor C1 fails open:

• Transformer (”coil”) T1 primary winding fails open:

• Transformer (”coil”) T1 secondary winding fails open:

For each of these conditions, explain why the resulting effects will occur.file 03712

21

Answer 14

• Transistor Q1 fails open (collector-to-emitter): No current through T1 primary or secondary, no high-voltage pulses across T1 secondary, full 12 volts (constant) across C1, voltage pulses still seen across L1,no spark at spark plug.

• Transistor Q1 fails shorted (collector-to-emitter): Constant current through T1 primary, no high-voltagepulses across T1 secondary, nearly 0 volts across C1, very weak voltage pulses across L1, no spark atspark plug.

• Reluctor magnet weakens: Smaller voltage pulses across L1, smaller current pulses through T1 primary,smaller voltage pulses across T2 secondary, weak or no spark at spark plug.

• Capacitor C1 fails shorted: Constant current through T1 primary, no high-voltage pulses across T1

secondary, nearly 0 volts across C1, normal voltage pulses across L1, no spark at spark plug.

• Capacitor C1 fails open: Excessive voltage pulses seen at Q1 collector (with respect to ground), veryrapid failure of Q1.

• Transformer (”coil”) T1 primary winding fails open: No current through T1 primary or secondary, nohigh-voltage pulses across T1 secondary, zero volts (constant) across C1, voltage pulses still seen acrossL1, no spark at spark plug.

• Transformer (”coil”) T1 secondary winding fails open: All voltages and currents fairly normal except forno voltage across T1 secondary and no spark at spark plug, perhaps slightly greater voltage pulses seenat Q1 collector with respect to ground.

Notes 14

The purpose of this question is to approach the domain of circuit troubleshooting from a perspective ofknowing what the fault is, rather than only knowing what the symptoms are. Although this is not necessarilya realistic perspective, it helps students build the foundational knowledge necessary to diagnose a faultedcircuit from empirical data. Questions such as this should be followed (eventually) by other questions askingstudents to identify likely faults based on measurements.

22

Question 15

Predict how all component voltages and currents in this circuit will be affected as a result of the followingfaults. Consider each fault independently (i.e. one at a time, no multiple faults):

Chassisground

"Coil"

Sparkplug

Ignition switch

12 VDC

N S

Reluctor

Q1

C1

T1

L1

V1 Q2



• Capacitor C1 fails shorted:

• Capacitor C1 fails open:

• Reluctor coil L1 fails open:


23

Answer 15

• Transistor Q2 fails open (collector-to-emitter): No current through T1 primary or secondary, no high-voltage pulses across T1 secondary, full 12 volts (constant) across C1, no current through any terminalof Q1, voltage pulses still seen across L1, no spark at spark plug.

• Transistor Q1 fails shorted (collector-to-emitter): Constant current through T1 primary, no high-voltagepulses across T1 secondary, nearly 0 volts across C1, voltage pulses still seen across L1, no spark atspark plug.

• Capacitor C1 fails shorted: Constant current through T1 primary, no high-voltage pulses across T1

secondary, nearly 0 volts across C1, normal voltage pulses across L1, no spark at spark plug.

• Capacitor C1 fails open: Excessive voltage pulses seen at Q1 collector (with respect to ground), veryrapid failure of Q1 and possibly Q2.

• Reluctor coil L1 fails open: No current through T1 primary or secondary, no high-voltage pulses acrossT1 secondary, full 12 volts (constant) across C1, full 12 volts (constant) across Q2 collector-to-emitter,no voltage pulses seen across L1, no spark at spark plug.

Notes 15


24

Question 16

Choose the right type of bipolar junction transistor for each of these switching applications, drawingthe correct transistor symbol inside each circle:

Load

+V+V

Switch sourcing currentto transistor

Transistor sourcingcurrent to load

Load

+V

Transistor sinkingcurrent from loadSwitch sinking current

from transistor

file 02407

25

Answer 16

Load

+V+V


Transistor sourcingcurrent to load

Load

+V

Transistor sinkingcurrent from loadSwitch sinking current

from transistor

NPN

PNP

Follow-up question: explain why neither of the following transistor circuits will work. When thepushbutton switch is actuated, the load remains de-energized:

Load

+V+V

Load

+V

Notes 16

Discuss with your students the meaning of the words ”sourcing” and ”sinking” in case they are notyet familiar with them. These are very common terms used in electronics (especially digital and powercircuitry!), and they make the most sense in the context of conventional flow current notation.

In order for students to properly choose and place each transistor to make the circuits functional, theymust understand how BJTs are triggered on (forward-biasing of the base-emitter junction) and also whichdirections the currents move through BJTs. The two example circuits shown in this question are veryrealistic.

26

Question 17

Choose the right type of bipolar junction transistor for each of these switching applications, drawingthe correct transistor symbol inside each circle:

Load

+V

+V

Switch sourcing currentto transistor Load

+V

Switch sinking currentfrom transistor

Transistor sinkingcurrent from load

current to loadTransistor sourcing

Also, explain why resistors are necessary in both these circuits for the transistors to function withoutbeing damaged.

file 02408

27

Answer 17

Load

+V

+V


Load

+V

Switch sinking currentfrom transistor

Transistor sinkingcurrent from load

current to loadTransistor sourcing

NPN

PNP

Follow-up question: explain why neither of the following transistor circuits will work. When thepushbutton switch is actuated, the load remains de-energized:

Load

+V

+V

Load

+V

Notes 17

Discuss with your students the meaning of the words ”sourcing” and ”sinking” in case they are notyet familiar with them. These are very common terms used in electronics (especially digital and powercircuitry!), and they make the most sense in the context of conventional flow current notation.

In order for students to properly choose and place each transistor to make the circuits functional, theymust understand how BJTs are triggered on (forward-biasing of the base-emitter junction) and also whichdirections the currents move through BJTs. The two example circuits shown in this question are veryrealistic.

28

Question 18

In this circuit, the electric motor is supposed to turn on whenever the cadmium sulfide photocell isdarkened:

Mtr

Unfortunately, though, the motor refuses to turn on no matter how little light strikes the photocell.In an attempt to troubleshoot the circuit, a technician measures voltage between the collector and emitterterminals of the transistor with the photocell covered by a piece of dark tape, and measures full batteryvoltage. The technician also measures voltage between the collector and base terminals of the transistor,and measures full battery voltage. At that point, the technician gives up and hands the problem to you.

Based on this information, what do you suspect is faulty in this circuit, and how might you determinethe exact location of the fault? Also, identify what you know to be not faulted in the circuit, based on theinformation given here.

file 00501

Answer 18

The battery, and its connections to the rest of the circuit, are in good condition. Also, we know thatthe motor is not failed open. In all likelihood, the transistor is not being ”told” to turn on.

Notes 18

It is just as important for your students to be able to identify what is not faulted in a system as it isfor them to be able to identify what is faulted. Replacing components that are not faulted is expensive andwasteful!

An essential part of answering this question is what the photocell does when light strikes it. Obviously,it undergoes a change in electrical resistance, but which way? This is something your students will have todetermine before they can successfully troubleshoot the system. If they do not understand what the systemis supposed to do, they will be helpless in interpreting what it is presently doing.

29

Question 19

The following circuit used to work fine, but does not work as it should anymore. The lamp does notcome on, no matter what is done with the switch:

+V

Q1

Switch

R1

Lamp

TP1

TP2

TP3

TP4

The very first thing you do is use your multimeter to check for source voltage, TP4 to ground. Betweenthose points you find 24 volts DC, just as it should be. Next you measure voltage between TP2 and groundwith the switch pressed and unpressed. With the switch pressed, there is no voltage between TP2 andground, but there is voltage (about 23.5 volts) when the switch is unpressed.

From this information, determine at least two possible failures in the circuit which could cause the lampnot to energize, and also account for the voltage measurements taken.

file 03720

Answer 19

Possible circuit faults:

• Lamp burned out• Transistor failed open at collector terminal• Bad (open) connection between lamp and transistor• Bad (open) connection between lamp and ground

Follow-up question: explain why we may say with confidence that there is no problem with the resistoror the switch.

Notes 19

Discuss with your students how to reason from the empirical data to a set of possible faults in thecircuit, and also how certain areas of the circuit (or individual components) may be safely eliminated fromthe list of possible faults.

30

Question 20

What is the significance of a transistor’s beta (β) rating? What, exactly, does this figure mean? Explainhow you might set up an experiment to calculate the beta of a given transistor.

Obtain a copy of a datasheet for a small-signal transistor such as the 2N2222 and see what it has to sayabout that transistor’s beta. Note: the beta of a transistor may be represented by the Greek letter (β), orby a completely different symbolism known as a hybrid parameter: hFE

file 00482

Answer 20

βDC = IC

IB

βAC = ∆IC

∆IB

Notes 20

The ”beta” rating of a transistor is a very important, yet very elusive parameter. Discuss the meaningof this specification with your students, and also their experimental setups for empirically determining beta.Then, compare the results of this discussion with the figures obtained from their datasheets. What do thedatasheet figures indicate about the nature of beta as a parameter?

After sharing the datasheet results, re-discuss the experimental setups for measuring beta. Would asingle test be sufficient, or would it be advisable to test beta more than once for a given transistor?

31

Question 21

Explain what β means for a bipolar junction transistor, in your own words.file 02321

Answer 21

I’ll let you figure this one out for yourself!

Notes 21

Many written definitions for β may be found in various textbooks and lexicons. What I am looking forhere, as best as possible, is a definition cast in students’ own words.

32

Question 22

Write the mathematical formula defining β for a bipolar junction transistor:

β =

file 02320

Answer 22

β =IC

IB

Notes 22

This definition may be found in any introductory electronics textbook, so you should ask you studentsto explain something about this formula in order to make it a meaningful discussion question. Suitableexplanations to request would be:

• What are some typical values for β?• Is β always greater than 1 or less than 1? What does this suggest about the two transistor currents in

the formula?• What would the β value be for an ideal transistor?

33

Question 23

An easy way to increase the effective current gain of a transistor is to ”cascade” two of them in aconfiguration called a Darlington pair:

Darlington pair

Complete this schematic diagram, showing how a Darlington pair could be used to enable a cadmiumsulfide (CdS) photocell to turn a motor on and off:

Mtr

file 00956

Answer 23

Mtr

Notes 23

Ask your students to define ”gain” as it applies to a transistor circuit. Ask them to explain why aDarlington pair has a greater current gain than a single transistor, and why that trait is important in acircuit such as this.

34

Question 24

Calculate all labeled currents in this Darlington pair circuit (shown in this schematic in conventionalflow notation), assuming a typical forward base-emitter junction voltage drop of 0.7 volts for each transistor:

19 V

150 Ω

I1I2

I3

I4

β = 85

β = 30

file 02324

Answer 24

• I1 = 44.01 µA• I2 = 113.5 mA• I3 = 3.785 mA• I4 = 117.3 mA

Notes 24

Have your students come to the front of the class, either as individuals or as groups, and presentproblem-solving techniques for determining the unknown currents.

35

Question 25

Calculate all labeled currents in this Darlington pair circuit (shown in this schematic in conventionalflow notation), assuming a typical forward base-emitter junction voltage drop of 0.7 volts for each transistor:

I1

I2

I3

I4

β = 90

β = 25

100 Ω

48 V

file 02325

Answer 25

• I1 = 465.8 mA• I2 = 197.0 µA• I3 = 17.92 mA• I4 = 466.0 mA

Notes 25

Have your students come to the front of the class, either as individuals or as groups, and presentproblem-solving techniques for determining the unknown currents.

36

Question 26

An alternative to the Darlington pair is the Sziklai pair, formed by a complementary pair of bipolartransistors:

Darlington pair Sziklai pair

Complete the following circuits, showing how a switch would be connected to each of the transistor pairsto exert control over the electric motor:

MtrMtr

file 02049

37

Answer 26

MtrMtr

Follow-up question: why would anyone want to use either a Darlington or Sziklai pair when asingle transistor is able to switch current on its own? What advantage do either of these transistor pairconfigurations give over a single transistor?

Notes 26

Sziklai pair circuits are often not discussed in electronics texts until the subject of audio power amplifiers,where the Sziklai pair is offered as an alternative to complementary power transistors in a push-pull circuit(where the final output transistors are both NPN instead of one being NPN and the other PNP). There isnothing wrong, however, with introducing the Sziklai pair configuration in the context of a simple switchingcircuit as it is done here.

38

Question 27

Explain how the one toggle switch is able to switch both transistors on and off simultaneously in thismotor control circuit:

+V

Mtr

file 02410

Answer 27

The switch exerts direct control over the lower transistor, which then indirectly turns on the uppertransistor. When both transistors are turned on, the motor runs.

Notes 27

This question is really a precursor to analyzing the H-bridge motor drive circuit.

39

Question 28

Explain the operation of this ”H-bridge” motor control circuit:

Mtr

Q1 Q2

Q3 Q4

+V

R1 R2

+V

R3

Fwd Rvs

At any given moment, how many transistors are turned on and how many are turned off? Also, explainwhat would happen to the function of the circuit if resistor R1 failed open.

file 00449

Answer 28

Two transistors are on at any given time, and the other two are off. If R1 fails open, the motor will notbe able to go in the ”forward” (Fwd) direction.

Challenge question: what type of DC motor is this drive circuit designed for? Shunt-wound, series-wound, compound, or permanent magnet? Explain your answer.

Notes 28

The ”H-drive” circuit is a very common method of reversing polarity to a DC motor (or other polarity-sensitive load), using only a single-pole switch. Very, very large electric motor ”drives” have been based onthis same design.

40

Question 29

Predict how the motor function in this circuit will be affected as a result of the following faults. Considereach fault independently (i.e. one at a time, no multiple faults):

Mtr

Q1 Q2

Q3 Q4

+V

R1 R2

+V

R3

Fwd Rvs





• Resistor R1 fails open:






41

Answer 29

• Transistor Q1 fails open (collector-to-emitter): Motor fails to turn in ”reverse” direction, can still turnin ”forward” direction.

• Transistor Q2 fails open (collector-to-emitter): Motor fails to turn in ”forward” direction, can still turnin ”reverse” direction.

• Transistor Q3 fails open (collector-to-emitter): Motor fails to turn in ”forward” direction, can still turnin ”reverse” direction.

• Transistor Q4 fails open (collector-to-emitter): Motor fails to turn in ”reverse” direction, can still turnin ”forward” direction.

• Resistor R1 fails open: Motor fails to turn in ”forward” direction, can still turn in ”reverse” direction.

• Resistor R2 fails open: Motor fails to turn in ”reverse” direction, can still turn in ”forward” direction.

• Resistor R3 fails open: Motor cannot turn in either direction.

• Transistor Q3 fails shorted (collector-to-emitter): Motor turns in ”forward” direction even when theswitch is in the center (off) position.

• Transistor Q4 fails shorted (collector-to-emitter): Motor turns in ”reverse” direction even when theswitch is in the center (off) position.

Notes 29


42

Question 30

The circuit shown here is part of a digital logic gate circuit:

VCC

Input

Output

VCC

Logic circuits operate with their transistors either fully ”on” or fully ”off,” never in-between. Determinewhat state the LED will be in (either on or off) for both switch positions. You may find it helpful to tracecurrents and label all voltage drops in this circuit for the two switch states:

VCC

VCC

Switch down

VCC

VCC

Switch up

For your voltage drop calculations, assume the following parameters:

• VCC = 5 volts• VBE (conducting) = 0.7 volts• VCE (conducting) = 0.3 volts• Vf (regular diode conducting) = 0.7 volts• Vf (LED conducting) = 1.6 volts

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Answer 30

Switch down, LED on; switch up, LED off.

Notes 30

The circuit shown in this question is a partial TTL inverter gate. I opted to simplify the circuit (omittingthe ”steering” diodes usually found at the input) for the sake of simplicity, so students could concentratetheir attention on the two transistor stages following. Although this circuit may appear intimidating, it isnot that difficult to trace currents and calculate voltage drops if one approaches it methodically.

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Question 31

One of the simplest transistor ”logic” circuits used in computer circuitry is the so-called inverter gate.Its logic diagram symbol is this:

"Inverter" logic gate

Vcc

Input Output

Inverters are often bundled six to a ”DIP” (Dual-Inline Package) module, where all gates share the sametwo power supply connections (”Vcc” and ”Ground”), like this:

14 13 12 11 10 9 8

7654321

Vcc

GND

5404/7404Hex inverter

When constructed with bipolar junction transistors, the internal schematic diagram of a single invertergate looks something like this:

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Output

Input

Vcc

Q2

Q3

R1 R2

R3

R4

Vcc

Vcc = 5 volts

Load

Internal diagram of a TTL"inverter" logic gate

Q1

D1 D2D3

Being a digital logic circuit, it only ”understands” two states: on and off. As you can see, a SPDTswitch provides signal input into the logic gate, either full supply voltage (VCC) or no voltage at all (Gnd).

Determine the voltage across the load resistance for each of these switch states. Based on your analysis,what is the logical function of an ”inverter” gate?

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Answer 31

An ”inverter” gate simply reverses the logic state of the input.

Notes 31

There is much to analyze in this circuit, and this question is not intended to be a full introduction todigital logic. It does, however, provide a preview of things to come, as well as an excellent opportunity toanalyze diode and transistor circuitry in the simplest possible way: either fully ”on” or fully ”off”.

One tactic I have found useful as an instructor for analyzing complex circuits in front of a class isto project the image onto a whiteboard using a bright computer projector. Then, you may use dry-erasemarkers on the whiteboard to ”comment” the diagram with voltage and current notations as you proceed toanalyze the circuit with your students. Analyzing the circuit for a different logic state is easy: merely erasethe ”comments” made previously, and the schematic itself is unchanged, ready to be ”marked up” again.

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Question 32

There is a problem somewhere in this electronic ignition circuit. The ”coil” does not output high voltageas it should when the reluctor spins. A mechanic already changed the coil and replaced it with a new one,but this did not fix the problem.

Chassisground

"Coil"

12 VDC

N S

Reluctor

TP1 TP2

TP3

TP4

Ignitionswitch

You are then asked to look at the circuit to see if you can figure out what is wrong. Using yourmultimeter, you measure voltage between TP1 and ground (12 volts DC) and also between TP2 and ground(12 volts DC). These voltage readings do not change at all as the reluctor spins.

From this information, identify two possible faults that could account for the problem and all measuredvalues in this circuit, and also identify two circuit elements that could not possibly be to blame (i.e. twothings that you know must be functioning properly, no matter what else may be faulted). The circuitelements you identify as either possibly faulted or properly functioning can be wires, traces, and connectionsas well as components. Be as specific as you can in your answers, identifying both the circuit element andthe type of fault.

• Circuit elements that are possibly faulted

1.

2.

• Circuit elements that must be functioning properly

1.

2.

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Answer 32

Note: the following answers are not exhaustive. There may be more circuit elements possibly at faultand more circuit elements known to be functioning properly!

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• Circuit elements that are possibly faulted1. Transistor failed open between collector and emitter2. Reluctor coil has failed open

• Circuit elements that must be functioning properly1. Battery2. Ignition switch

Notes 32

Ask your students to identify means by which they could confirm suspected circuit elements, bymeasuring something other than what has already been measured.

Troubleshooting scenarios are always good for stimulating class discussion. Be sure to spend plentyof time in class with your students developing efficient and logical diagnostic procedures, as this will assistthem greatly in their careers.

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Question 33

There is a problem somewhere in this electronic ignition circuit. The ”coil” does not output high voltageas it should when the reluctor spins. A mechanic already changed the coil and replaced it with a new one,but this did not fix the problem.

Chassisground

"Coil"

12 VDC

N S

Reluctor

TP1 TP2

TP3

TP4

Ignitionswitch

You are then asked to look at the circuit to see if you can figure out what is wrong. Using yourmultimeter, you measure voltage between TP1 and ground (12 volts DC) and also between TP2 and ground(0 volts DC). These voltage readings do not change at all as the reluctor spins.

From this information, identify two possible faults that could account for the problem and all measuredvalues in this circuit, and also identify two circuit elements that could not possibly be to blame (i.e. twothings that you know must be functioning properly, no matter what else may be faulted). The circuitelements you identify as either possibly faulted or properly functioning can be wires, traces, and connectionsas well as components. Be as specific as you can in your answers, identifying both the circuit element andthe type of fault.

• Circuit elements that are possibly faulted

1.

2.

• Circuit elements that must be functioning properly

1.

2.

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Answer 33

Note: the following answers are not exhaustive. There may be more circuit elements possibly at faultand more circuit elements known to be functioning properly!

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• Circuit elements that are possibly faulted1. Transistor failed shorted between collector and emitter2. Capacitor failed shorted

• Circuit elements that must be functioning properly1. Battery2. Ignition switch

Notes 33

Ask your students to identify means by which they could confirm suspected circuit elements, bymeasuring something other than what has already been measured.

Troubleshooting scenarios are always good for stimulating class discussion. Be sure to spend plentyof time in class with your students developing efficient and logical diagnostic procedures, as this will assistthem greatly in their careers.

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Question 34

Don’t just sit there! Build something!!

Learning to mathematically analyze circuits requires much study and practice. Typically, studentspractice by working through lots of sample problems and checking their answers against those provided bythe textbook or the instructor. While this is good, there is a much better way.

You will learn much more by actually building and analyzing real circuits, letting your test equipmentprovide the ”answers” instead of a book or another person. For successful circuit-building exercises, followthese steps:

1. Carefully measure and record all component values prior to circuit construction, choosing resistor valueshigh enough to make damage to any active components unlikely.

2. Draw the schematic diagram for the circuit to be analyzed.3. Carefully build this circuit on a breadboard or other convenient medium.4. Check the accuracy of the circuit’s construction, following each wire to each connection point, and

verifying these elements one-by-one on the diagram.5. Mathematically analyze the circuit, solving for all voltage and current values.6. Carefully measure all voltages and currents, to verify the accuracy of your analysis.7. If there are any substantial errors (greater than a few percent), carefully check your circuit’s construction

against the diagram, then carefully re-calculate the values and re-measure.

When students are first learning about semiconductor devices, and are most likely to damage themby making improper connections in their circuits, I recommend they experiment with large, high-wattagecomponents (1N4001 rectifying diodes, TO-220 or TO-3 case power transistors, etc.), and using dry-cellbattery power sources rather than a benchtop power supply. This decreases the likelihood of componentdamage.

As usual, avoid very high and very low resistor values, to avoid measurement errors caused by meter”loading” (on the high end) and to avoid transistor burnout (on the low end). I recommend resistors between1 kΩ and 100 kΩ.

One way you can save time and reduce the possibility of error is to begin with a very simple circuit andincrementally add components to increase its complexity after each analysis, rather than building a wholenew circuit for each practice problem. Another time-saving technique is to re-use the same components in avariety of different circuit configurations. This way, you won’t have to measure any component’s value morethan once.

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Answer 34

Let the electrons themselves give you the answers to your own ”practice problems”!

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Notes 34

It has been my experience that students require much practice with circuit analysis to become proficient.To this end, instructors usually provide their students with lots of practice problems to work through, andprovide answers for students to check their work against. While this approach makes students proficient incircuit theory, it fails to fully educate them.

Students don’t just need mathematical practice. They also need real, hands-on practice building circuitsand using test equipment. So, I suggest the following alternative approach: students should build theirown ”practice problems” with real components, and try to mathematically predict the various voltage andcurrent values. This way, the mathematical theory ”comes alive,” and students gain practical proficiencythey wouldn’t gain merely by solving equations.

Another reason for following this method of practice is to teach students scientific method: the processof testing a hypothesis (in this case, mathematical predictions) by performing a real experiment. Studentswill also develop real troubleshooting skills as they occasionally make circuit construction errors.

Spend a few moments of time with your class to review some of the ”rules” for building circuits beforethey begin. Discuss these issues with your students in the same Socratic manner you would normally discussthe worksheet questions, rather than simply telling them what they should and should not do. I nevercease to be amazed at how poorly students grasp instructions when presented in a typical lecture (instructormonologue) format!

A note to those instructors who may complain about the ”wasted” time required to have students buildreal circuits instead of just mathematically analyzing theoretical circuits:

What is the purpose of students taking your course?

If your students will be working with real circuits, then they should learn on real circuits wheneverpossible. If your goal is to educate theoretical physicists, then stick with abstract analysis, by all means!But most of us plan for our students to do something in the real world with the education we give them.The ”wasted” time spent building real circuits will pay huge dividends when it comes time for them to applytheir knowledge to practical problems.

Furthermore, having students build their own practice problems teaches them how to perform primaryresearch, thus empowering them to continue their electrical/electronics education autonomously.

In most sciences, realistic experiments are much more difficult and expensive to set up than electricalcircuits. Nuclear physics, biology, geology, and chemistry professors would just love to be able to have theirstudents apply advanced mathematics to real experiments posing no safety hazard and costing less than atextbook. They can’t, but you can. Exploit the convenience inherent to your science, and get those studentsof yours practicing their math on lots of real circuits!

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Bipolar junction transistors as switches - ibiblio · Bipolar junction transistors as switches ... Some of the following transistor switch circuits are properly conﬁgured, and some

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